An LTE network is designed to support UEs from different 3GPP releases, i.e. Rel-8/9/10/11, in a backward compatible way. One of the LTE network design objective is to enable co-scheduling of such UEs in time, frequency and space (Multiple User—Multiple Input Multiple Output, MU-MIMO) dimensions with as few scheduling constraints as possible.
Furthermore, the LTE standard should be able to support various and flexible deployments. Some examples of expected deployments for modern LTE networks (Rel-11 and beyond) include, e.g.:                Macro-deployments, where large cells are typically divided into independent sectors,        HetNet-deployments, where pico-cells are deployed within the coverage of macro-cells in order, e.g., to improve coverage for high data rate UEs, and        Hotspot scenarios where an access point serves a small area with high throughput need.        
A “cell” is characterized in LTE by a “cell-ID” and the carrier frequency, which affects several cell-specific algorithms and procedures. A cell is a coverage area of a Radio Base Station, RBS, or eNodeB. An RBS or eNodeB may be associated with a plurality of cells.
The UL of LTE is designed assuming coherent processing, i.e., the receiver is assumed to be able to estimate the radio channel from a transmitting UE and to take advantage of such information in the detection phase. Therefore, each transmitting UE sends a Reference Signal, RS, associated to each UL data channel, i.e. the Physical Uplink Shared Channel, PUSCH.
RSs from different UEs within the same cell potentially interfere with each other and, assuming synchronized networks, even with RSs originated by UEs in neighbouring cells. In order to limit the level of interference between RSs, different techniques have been introduced in different LTE releases in order to allow orthogonal or semi-orthogonal RSs. The design principle of LTE assumes orthogonal RSs within each cell and semi-orthogonal RS among different cells (even though orthogonal RSs can be achieved for aggregates of cells by so called “sequence planning”).
Each RS is characterized by a group-index and a sequence-index, which define the so called base-sequence. Base sequences are cell-specific in Rel-8/9/10 and they are a function of the cell-ID. Different base sequences are semi-orthogonal. The RS for a given UE is only transmitted on the same bandwidth of Physical Uplink Shared Channel, PUSCH, and the base sequence is correspondingly generated so that the RS signal is a function of the PUSCH bandwidth. One example is illustrated in FIG. 1, where DMRS 1 and DMRS 2 represent respective Demodulation Reference Signal, DMRS, of different UEs. For each subframe, 2 RSs are transmitted, one per slot, as indicated in FIG. 2.
On top of the base sequence, a phase shift, CS, is applied in frequency domain and an orthogonal cover code, OCC, is applied in time domain over the slots. Orthogonal RS can be achieved by use of CS in Rel-8/9 or by CS in conjunction with OCC in Rel-10 and later releases.
CS is a method to achieve orthogonality based on cyclic time shifts (which correspond to phase rotations in frequency domain), under certain propagation conditions, among RSs generated from the same base sequence. Only 8 different CS values can be signalled by scheduling grants in Rel-8/9/10, even though in practice less than 8 orthogonal RS can be achieved depending on channel propagation properties. Even though CS is effective in multiplexing RSs assigned to fully overlapping bandwidths, orthogonality is lost when the bandwidths differ and/or when the interfering UEs employ another base sequence.
In order to increase interference randomization, a pseudo-random offset to the CS values is applied (CS hopping). The randomization pattern is cell-specific up to Rel-10 and UE specific in Rel-11. A different CS offset is in general applied in each slot and it is known at both UE and RBS/eNodeB sides, so that it can be compensated at the receiver side during channel estimation. The pseudo-random CS offset is combined with the signalled UE-specific CS offset for each slot, and a modulo 12 operation is performed in order to avoid exceeding the maximum phase rotation speed. CS randomization is always enabled and generates random cell-specific CS offsets per slot. The pseudo-random CS pattern is a function of the cell-ID and is thus cell-specific.
OCC is a multiplexing technique based on orthogonal time domain codes, operating on the 2 RSs provided for each UL subframe. The OCC code [1-1] is able to suppress an interfering RS as long as its contribution after the RBS/eNodeB matched filter is identical on both RSs of the same subframe. Similarly, the OCC code [1 1] is able to suppress an interfering RS as long as its contribution after the RBS/eNodeB matched filter has opposite sign respectively on the two RSs of the same subframe.
While base-sequences are assigned in a semi-static fashion, CS and OCC are UE specific and dynamically assigned as part of the scheduling grant for each UL PUSCH transmission.
One of the main innovations in the UL for LTE Rel-10 is the introduction of Multi-Antenna techniques which can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas. This results in a multiple-input multiple-output, MIMO, communication channel and such systems and/or related techniques are commonly referred to as MIMO.
LTE Rel.10 supports a spatial multiplexing mode (single-user MIMO or SU-MIMO) in the communication from a single UE to the RBS/eNodeB. SU-MIMO is aimed for high data rates in favourable channel conditions. SU-MIMO consists of the simultaneous transmission of multiple data streams on the same bandwidth, where each data stream is usually termed as a layer. Multi-antenna techniques, such as linear precoding, are employed at the transmitter in order to differentiate the layers in the spatial domain and allow the recovering of the transmitted data at the receiver side. Typically, an individual demodulation reference signal, DMRS, is transmitted for each transmission layer. Alternatively, an individual Sounding Reference Signal, SRS, may be transmitted for each transmit antenna, e.g., for channel sounding purposes.
Another MIMO technique supported by LTE Rel.10 is MU-MIMO, where multiple UEs belonging to the same cell are completely or partly co-scheduled on the same bandwidth and time slots. Each UE in the MU-MIMO configuration may possibly transmit multiple layers, thus operating in SU-MIMO mode. In order to achieve good performance, DMRS for the co-scheduled UEs may be orthogonalized for MU-MIMO applications. One possible means for obtaining orthogonality is to apply OCCs.
One possible improvement to LTE DMRS is to apply IFDMA, which has been discussed in 3GPP contribution R1-100262, “Analysis and evaluation of UL DM RS design for LTE-A scenarios”. IFDMA is a multiplexing technique for OFDM signals, consisting of an interleaved mapping of the signal to the subcarriers at the input of the Inverse Fast Fourier Transform, IFFT, OFDM modulator at the transmitter. The signal is mapped to each L:th subcarrier in a comb fashion, where L is the IFDMA order. Corresponding demapping is performed at the receiver side. With IFDMA up to L L-order UEs may be multiplexed on overlapping bandwidth, as longs as each UE is assigned a different subcarrier offset in the comb mapping, so that its signal does not overlap in frequency domain with the signal from other UEs. FIG. 3 is a schematic illustration of IFDMA RS multiplexing of two IFDMA enabled UEs, UE1 and UE2 applying IFDMA of order 2.
OCC may be applied to the IFDMA modulated DMRS in the slots of a subframe.
In case new UEs supporting IFDMA are introduced in an existing network, a problem of compatibility with existing non-IFDMA UEs arises. In order to achieve orthogonal DMRS between new and non-IFDMA (legacy) UEs, a candidate solution is to employ OCC. However, due to CS hopping patterns, OCC is not effective in this case and orthogonality may not be achieved.